Snowmobiles are driven by endless drive tracks supported and tensioned by rear suspension assemblies. Many such rear suspension assemblies have front and rear suspension arms and one or more shock absorbers that are connected between the tunnel of the snowmobile and a slide frame of the rear suspension assembly. The manner in which the slide frame, and therefore the track, moves over bumps and recesses as the snowmobile is in movement depends of the geometry of the rear suspension assembly.
Some geometries result in what is known as an uncoupled rear suspension assembly. In an uncoupled rear suspension assembly, the front and rear portions of the slide frame move independently from each other. For example, when the snowmobile moves forward and the rear suspension assembly encounters a bump, the front portion of the slide frame moves toward the tunnel while the rear portion of the slide frame will not move toward the tunnel until it also encounters the bump. In an uncoupled rear suspension assembly, the slide frame can pivot relative to the tunnel as seen from the side of the snowmobile.
Some other geometries result in what is known as a coupled rear suspension assembly. In a coupled rear suspension assembly, the front and rear portions of the slide frame move substantially together. For example, when the snowmobile moves forward and the rear suspension assembly encounters a bump, the front portion of the slide frame moves toward the tunnel and also pulls the rear portion of the slide frame toward the tunnel even though the rear portion of the of the slide frame has not yet encountered the bump. In a coupled rear suspension assembly, the slide frame moves essentially linearly relative to the tunnel without substantially altering its orientation relative to the tunnel.
As both coupled and uncoupled rear suspension assemblies have advantages, some snowmobiles have a rear suspension assembly that acts like an uncoupled rear suspension assembly from a fully extended position of the rear suspension assembly to a predetermined intermediate position of the rear suspension assembly and acts like a coupled rear suspension assembly from the predetermined intermediate position of the rear suspension assembly to a fully compressed position of the rear suspension assembly. In some of these rear suspension assemblies, stoppers are provided such that a portion of the rear suspension assembly abuts these stoppers when the rear suspension assembly reaches the predetermined intermediate position as it moves toward the fully compressed position. When this happens, the stoppers reduce the degrees of freedom of movement of the rear suspension assembly suddenly changing the rear suspension assembly from being an uncoupled rear suspension assembly to being a coupled rear suspension assembly. United States Patent RE38,124, reissued May 27, 2003, and U.S. Pat. No. 6,206,124, issued Mar. 27, 2001, the entirety of both of which is incorporated herein by reference, describe rear suspension assemblies of this type. However this sudden change can be felt by the driver and, if applicable, the passenger(s) of the snowmobile which can be a source of mild discomfort or annoyance. Also, the impacts with the stoppers require that some of the parts of the rear suspension assembly need to be large and robust enough to resist the repeated impacts.
There is therefore a need for a rear suspension assembly for a snowmobile that can vary the degree of coupling of the rear suspension assembly.